8255(DOCS)		Optical Printer Control System		8255(DOCS)

NAME
    8255 - how to control 8255 based digital I/O cards

8255 PORTS
    The 8255 family of chips (82C55, etc) are usually one chip
    solutions to getting 24 bits of programmable digital I/O.

    Typically, the chip is configured at a base I/O address, 
    such as 0310. 
    
    There are four ports (base+0, base+1, base+2 and base+3)
    that are used to control the chip:

    	base+0    - PORT #A
	base+1    - PORT #B
	base+2    - PORT #C
	base+3    - CONTROL

8255 PROGRAMMING
    The control byte controls the I/O direction of the 3 ports,
    and breaks out as follows:

        Bit   Description
	---   --------------------------------------------------
	 0    Port C (low 4 bits): 1=input, 0=output
	 1    Port B (all 8 bits): 1=input, 0=output
	 2    Mode selection: 0=MODE#0, 1=MODE#1
	 3    Port C (hi 4 bits): 1=input, 0=output
	 4    Port A (all 8 bits): 1=input, 0=output
            _
	 5   |_ 00 = MODE#0 (basic I/O)
	 6  _|  01 = MODE#1 (strobed I/O)
	        1x = MODE#2 (bidirectional bus)

	 7    Mode set flag (1=active, 0=normal)

    No initialization is 'required' to achive 24 bits of input;
    it's the default. During reset, all ports are programmed to be inputs. 
    
    Mainly, the control byte is the important factor. Here are some
    example values for the control byte:

		0x80	- A,B,C outputs
		0x9b	- A,B,C inputs
		0x92	- A+B input, C=output

8255 PROGRAMMING EXAMPLES
    Here's a C programming example that shows how to setup the 8255
    such that A+B are inputs, and C is outputs:

	/* INITIALIZATION */
	base = 0x310;
	out(base+3), 0x92;	/* A+B=in, C=out */

	/* READ/WRITE */
	a_val = inp(base+0);	/* read A */
	b_val = inp(base+1);	/* read B */
	out(base+2, c_val); 	/* write C */

   Here's an example showing a similar 8255 programming example
   in the OPCS system's HOMEDEFS.HOM file, used by the HOME program:

	# homedefs.hom
	start init_8255
	{
	    outport 0310 92
	    portset? 0310 01
	    {
	        print "Bit #1 set on port A"
	    }
	    portset? 0311 01
	    {
	        print "Bit #1 set on port B"
	    }
	    setbit 0312 01
	}
	end init_8255

8255 CHIP PINOUT
    The 82C55A is most commonly found in a 40 pin DIP.

			  82C55A
			 Top View
                       _____  _____
                      |     --     |
                PA3  [| 1       40 |]  PA4
                PA2  [| 2       39 |]  PA5
                PA1  [| 3       38 |]  PA6
                PA0  [| 4       37 |]  PA7
                 RD  [| 5       36 |]  WR
                 CS  [| 6       35 |]  RESET
                GND  [| 7       34 |]  D0
                 A1  [| 8       33 |]  D1
                 A0  [| 9       32 |]  D2
                PC7  [| 10      31 |]  D3
                PC6  [| 11      30 |]  D4
                PC5  [| 12      29 |]  D5
                PC4  [| 13      28 |]  D6
                PC0  [| 14      27 |]  D7
                PC1  [| 15      26 |]  VCC
                PC2  [| 16      25 |]  PB7
                PC3  [| 17      24 |]  PB6
                PB0  [| 18      23 |]  PB5
                PB1  [| 19      22 |]  PB4
                PB2  [| 20      21 |]  PB3
		      |____________|

8255 PORT MONITOR PROGRAM
    The OPCS software comes with 8255.exe, a program that monitors
    the realtime status of the 8255's I/O ports. Run '8255.exe'.
    This tool can be downloaded from http://seriss.com/opcs/ftp/

SEE ALSO
    CIODIO24(DOCS) - Docs on the 8255 based Digital I/O card

ORIGIN
    Gregory Ercolano, Topanga, California 04/12/00